Fuel systems for engines

ABSTRACT

A fuel system for an engine has an electronic governor controlling a pump, the governor receiving a number of signals including one signal from a transducer meansuring engine speed. This transducer includes a pump circuit, and is characterised in that the capacitor of the pump circuit is connected across the input and output of an operational amplifier.

United States Patent Williams et a1.

FUEL SYSTEMS FOR ENGINES Inventors: Malcolm Williams, Solihull;

Geoffrey Albert Kenyon Brunt, Glastonbury; Christopher Robin Jones,Alcester, all of England C.A.V. Limited, London, England Assignee:

Filed: Apr. 4, 1973 Appl. No.: 347,729

Foreign Application Priority Data Field of Search 123/139 E, 32 EA, 32AE, 123/102; 60/3928; 73/398; 307/247, 266, 268; 328/165, 158

[56] References Cited UNITED STATES PATENTS 3,478,512 11/1969 Brahm123/32 EA 3,695,242 10/1972 Tada 1 23/32 EA 3,699,935 10/1972 Adler123/102 3,717,174 12/1973 Butscher 123/32 EA 3,724,430 4/1973 Adler123/32 EA 3,724,433 4/1973 Voss 123/32 EA Primary ExamIneP-CharIes J.Myhre Assistant ExaminerRonald B. Cox Attorney, Agent, or Firm1-1o1man &Stern [57] ABSTRACT A fuel system for an engine has an electronicgovernor controlling a pump, the governor receiving a number of signalsincluding one signal from a transducer meansuring engine speed. Thistransducer includes a pump circuit, and is characterised in that thecapacitor of the pump circuit is connected across the inpu and output ofan operational amplifier.

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46a Sla 82a 9 la 1 FUEL SYSTEMS FOR ENGINES This invention relates tofuel systems for engines, particularly, but not exlcusively,compression-ignition engines. The invention further resides inelectronic pump circuits for use in such systems and for other purposes.

In one aspect, the invention resides in a fuel system for an engine,comprising in combination a pump sup plying fuel to the engine, anactuator controlling the pump, and an electronic governor forcontrolling the actuator, said governor receiving electrical signalsrepresenting engine speed and at least one further engine parameter, thespeed signal being obtained using a transducer producing an 21.0. outputat a frequency proportional to engine speed, an a pump circuit forconverting said signal to a do signal, said pump circuit including acapacitor across which is developed a voltage proportional to thefrequency of the ac. output, said capacitor being connected across theinput and output terminals of an operational amplifier.

In another aspect, the invention resides in an electronic pump circuitin which a voltage dependent on the frequency of an ac. input isdeveloped across a capacitor, characterised in that the capacitor isconnected across the input and output terminals of an operationalamplifier.

In the accompanying drawings,

FIG. 1 is a circuit diagram, partly in block form, illustrating one formof fuel system with which the invention can be used,

FIGS. 2 to 4 are graphs illustrating the outputs of three transducersused in FIG. 1,

FIG. 5 represents a fuel-speed characteristic for an engine to becontrolled by the arrangement of FIG. 1,

FIG. 6 is a view similar to FIG. 1 of a second form of fuel system,

FIG. 7 is a view similar to FIG. 5 but showing the characteristicobtained by FIG. 6,

FIG. 8 is a circuit diagram illustrating one example of the pulseshaping circuit used in FIG. 1 or FIG. 6,

FIG. 9 is a circuit diagram illustrating a modification of thearrangement shown in FIG. 8,

FIG. 10 is a circuit diagram illustrating one form of pump circuit,

FIG. 11 is a circuit diagram illustrating another form of pump circuit,

FIG. 12 is a circuit diagram of a third form of pump circuit, and

FIGS. 13 to are wave forms illustrating the operation of FIG. 12.

The examples described relate to a fuel injection system for a dieselengine driving a road vehicle, so that demand is set by an acceleratorpedal. However, the arrangements shown can be used with other engines,and the engine employed need not drive a road vehicle, in which case thedemand is of course set in some other way.

Referring first to FIG. 1, a fuel pump 11 supplies fuel to the cylindersof an engine 12 in turn, the fuel pump being driven in a conventionalmanner. with the timing of injection controlled in the usual way. Thedriving of the fuel pump forms no part of the present invention and isnot therefore described. Moreover, the type of pump used is notcritical, but in the example shown the pump is a conventional in-linepump having a control rod 14 the axial position of which determines therate of supply of fuel to the engine 12 by the pump 11. The axialposition of the control rod 14 is controlled by an electro-mechanicalactuator 13 to determine the pump output.

The system further includes three transducers 15a, 16 and 17. Thetransducer 15a produces an output having a frequency proportional toengine speed, this output being fed by way of a pulse shaping network15b to a pump circuit 15c which produces an output in the form of avoltage shown in FIG. 2, the magnitude of the voltage being dependent onthe rotational speed of the engine. The transducer 16 produces an outputvoltage shown in FIG. 3 the voltage being dependent on the rate ofsupply of fuel to the engine, (i.e. the pump output). For this purposethe transducer 16 conveniently senses the axial position of the controlrod 14 as indicated by the dotted line. The transducer 17 produces avoltage representing demand. Typically, the transducer 17 is controlledby the accelerator pedal of vehicle which is driven by the engine, andin the particular example being described, the engine is controlled byan all-speed governor, so that the output from the transducer 17 is avoltage representing demanded engine speed. The form of this voltage isshown in FIG. 4, and it should be noted that the slope of this output isopposite to the slopes of the outputs from the transducers 15. 16.

The outputs from the transducers 15, 16 and 17 are all applied, by wayof resistors 15d, 16a, 17a converting the signals to current signals, tothe inverting termin: l of an operational amplifier 18 connected as asumming amplifier, whilst the output from the transducer 16 is alsoconnected through a resistor 16b to the inverting terminal of anoperational amplifier 19 connected as a summing amplifier. The amplifierl8 and 19 are powered by positive and negative supply lines 21, 22 andhave their non-inverting terminals connected to a line 23 which is keptat a reference potential mid-way between the potentials of the lines 21,22. The origin of FIGS. 2 to 4 is the potential of the line 23, and thesupply for the power lines is derived from the vehicle battery.

The output from the amplifier 18 is fed through a diode 24 to a drivecircuit 25 which incorporates a power amplifier and which serves tocontrol the electro-mechanical actuator 13. Similarly, the outputtermnal of the amplifier 19 is connected to the drive circuit 25 througha diode 26. The diodes 24 and 26 together constitute a discriminator,which ensures that only the amplifier 18, 19 producing the more positiveoutput is coupled to the drive circuit 25 at any given instant. Thus, ifthe amplifier 18 is producing the more positive output, then the diode26 is reverse biased, and if the amplifier 19 is producing the morepositive output, the diode 24 is reverse biased. FIG. 1 also shows thefeedback resistors 27, 28 associated with the amplifiers 18, 19respectively, and it will be noted that the feedback circuit for eachamplifier is taken from the input terminal of the drive circuit 25. Byvirtue of this arrangement, the effective forward voltage drop acrossthe diodes 24 and 26 is reduced by a factor dependent upon the amplifieropen-loop gain, and so the temperature characteristics of the diodesbecome negligible when considering the temperature characteristics ofthe system. Also, there is a very sharp changeover from control by oneamplifier to contol by the other amplifier.

The basic operation is as follows. The amplifier 18 compares the inputcurrents it receives and modifies the pump output until the sum of theinput currents is zero.

The amplifier 19 receives a signal by way of the resistor 16brepresenting pump output and also receives a reference current from areference source 20. If the demanded pump output set by the amplifier 18exceeds a value set by the source 20, then the amplifier 19 produces anoutput which is more positive than the output of the amplifier 18, sothat the diode 24 ceases to conduct as previously explained and theamplifier 19 produces an output to the drive circuit 25. It should benoted that an increasing output from an amplifier 18 or 19 is in fact ademand for a decrease in fuel, that is to say there is an invertingstage between the amplifiers 18, 19 and the pump. When the amplifier 19is producing an output, the system operates in the same way as when theamplifier 18 is producing an output to reduce the output of theamplifier 19 to a value such that the output from the drive circuit 25keeps the control rod 14 in the position it has assumed. The system willstay in this condition until the amplifier 18 demands less fuel than themaximum set by the amplifier 19. When the amplifier l8 demands lessfuel, it produces a greater positive output than the amplifier 19, andso takes over the operation.

Referring now to FIG. 5, the way in which the governor is designed andoperates can be seen from the graph of pump output against speed. Thisgraph also shows the effect of a number of controls not yet mentioned inrelation to FIG. 1. The line 40 is set by the amplifier 18 by virtue ofthe way in which the comparison of actual and demanded speeds ismodified in accordance with the input from the transducer 16. The line40 in the drawings represents 50% demand, and is one of a family oflines stretching from demand to 100% demand. The extremes of thisfamily, that is to say no demand and full demand, are indicated at 38and 43. The line 38 is set by a current source 31 providing an input tothe inverting terminal of the amplifier 18, to ensure that the enginespeed varies with pump output in the manner indicated by the curve 38even when the demand is zero. The maximum speed is set by a control 29shown in FIG. 1 and which acts by limiting the maximum demand from thetransducer 17. The line 35 is the maximum fuel line which is set by theamplifier 19 as previously explained.

The boundary line 39 is a function of the engine, not the governor, andrepresents the no-load fuel requirements of the engine under differentdemands, so that the points 41 and 42 are the no-load engine speeds atzero and full demand, (i.e.) with the pedal released and fully depressedrespectively.

FIG. explains how the engine will behave in any circumstances. Supposethat the pedal has been set to demand 50%, corresponding to the line 40shown in FIG. 5. The exact position on the line 40 at any given instantwill depend upon the load on the engine, and so for this given settingof the pedal, the engine speed can vary within the limits set by thelines 35 and 40. The slope of the line 40 is, as previously explained, aresult of the input to the amplifier 18 from the transducer 16. Assumingthat the engine is operating at a particular point on the line 40, thenif the vehicle starts to go up an incline, the load will increase, andso for a given position of the pedal the operating point will move upthe line 40, so that the speed is reduced. If the load becomessufficiently great, the line 35 will be reached, and no further increasein pump output will be permitted. At this point, the speed fallsrapidly. If the load decreases, then the operating point moves down theline 40 with the corresponding increase in speed. If the load decreasesto zero, the line 39 is reached.

If the demand is changed, then assuming for the sake of argument that itchanges from 50% demand to demand, the pump output will increase asrapidly as the pump and governor will allow until the line 35 isreached, and the engine will then move along the line 35 onto themaximum demand line 43, and will assume a position on the line 43 whichis dependent upon the load.

If the demand is reduced, then assuming the demand is reduced from 50 to0%, the operating point will move downward until the fuel supply iszero. The speed then decreases until the line 38 is reached, after whichthe operating point moves up the line 38, finishing at a point on theline 38 determined by the load on the engme.

Turning now to FIG. 6, there is shown a second example in which thegovernor is a two-speed governor, that is to say a governor in which thedemand signal is a fuel signal which is compared with the actual fuel,the pump output then being modified to provide the desired fuel output.In FIG. 6, the amplifier 18 receives a signal from the transducer 16 byway of the resistor 16a, this signal representing actual fuel. A signalrepresenting demanded fuel is fed by way of the resistor 17a to theamplifier 18, but it will be noted that there is no speed term fed tothe amplifier 18 by way of the resistor 15d. The characteristics of thesystem are shown in FIG. 7. The line 40a is one of a family ofhorizontally extending lines which are set by the governor, and can betaken to represent the 50% demand line. When the pedal sets a demand of50%, the amplifier 18 sets the required fuel level. The operating pointon the line 40a will of course then depend on the load on the engine.

The amplifier l9 overrides the amplifier 18 in FIG. 6 in a similarmanner to the arrangement in FIG. 1, except that the amplifier 19 nowreceives a signal by way of the resistor 15d representing speed, andalso a reference current from a source 20a indicating the maximum enginespeed. The amplifier 19 sets the maximum speed of the engine, which isindicated by the line 43 in FIG. 7. It will be noted that the line 43has a slope, that is to say the maximum permitted speed varies with pumpoutput. This slope is obtained by feeding to the amplifier 19 a signalrepresenting pump output, this signal being fed by way of the resistor16b.

The maximum pump output, that is to say the line 35 in FIG. 7, is set bya control 29a which limits the maximum demand, in much the same way asthe control 29 limits the maximum speed in FIG. 1. The minimum enginespeed, indicated by the line 38, is set by a current source 31a, whichis similar to the current source 31 except that because the currentsource 31a acts on the amplifier 18, which does not receive a speedterm, the current source 31a must receive a speed term as indicated byits connection to the pump circuit 150.

FIG. 8 shows one form of the pulse shaping circuit 15b. Referring toFIG. 8, the circuit is powered by the supply lines 21, 22, 23. The inputto the circuit is taken from the transducer 15a and is fed between aninput terminal 45 and the line 23, and the output from the circuit istaken between a terminal 46 and the line 23.

The terminal 45 is connected to the line 23 through a resistor 47 and acapacitor 48 in series, the junction of the resistor 47 and capacitor 48being connected to the line 23 through a resistor 49 and a capacitor 51in series, and the junction of the resistor 49 and capacitor 51 beingconnected to the base of an n-p-n transistor 52. The transistor 52 hasits collector connected to the line 21, and its emitter connected to theemitter of a further n-p-n transistor 53, and to the collector of ann-p-n transistor 54. The emitter of the transistor 54 is connectedthrough a resistor 55 to the line 22, and the collector of thetransistor 53 is connected through a resistor 56 to the line 21. Thebase of the transistor 53 is connected to the line 23 through a resistor57. The lines 23 and 22 are further interconnected through a resistor 58and a pair of diodes 59, 61 in series, and the junction of the resistor58 and diode 59 is connected to the base of the transistor 54.

The collector of the transistor 53 is connected to the base of a p-n-ptransistor 62 having its emitter connected to the line 21 and itscollector connected to the output terminal 46, to the collector of thetransistor 53 through a capacitor 63, and through a pair of resistors 64and 65 in series to the line 22. The junction of the resistors 64 and 65is connected to the base of the transistor 53. Moreover, the terminal 46is connected to the collector of an n-p-n transistor 66, the emitter ofwhich is connected through a resistor 67 to the line 22 and the base ofwhich is connected through a diode 68 to the line 23.

As will be seen from FIG. 1, the output between the terminal 46 and theline 22 is applied to the pump circuit c, which can take a number offorms, but usually includes a pair of diodes. The purpose of thetransistor 66, the resistor 67 and the diode 68 is merely to provide twodiodes which in operation will compensate for the voltage drop acrossthe diodes in the pump circuit, so that the circuit is not affected bychanges in temperature.

The ac. signal from the transducer 15a is fed between the terminal 45and the line 23, and is filtered by the resistor-capacitor network 47,48, 49, 51. The transistors 52, 53, 54 and 62 with their associatedcomponents then produce a square wave between the terminal 46 and line22 in the following manner.

For ease of explanation, assume that the circuit is in one of its stablestates with the transistors 53, 54 and 62 conducting, the terminal 46approximately at the potential of the line 21. The potential at the baseof the transistor 53 at this stage is determined by the resistors 57, 64and 65. This is the state the circuit assumes with the voltage at theterminal 45 relatively low. As the voltage at the terminal 45 increases,the transistor 52 starts to conduct, and since the transistor 54 acts asa constant current source, current flow through the transistor 53 isreduced. Since the base current for the transistor 62 flows through thetransistor 53, conduction of the transistor 62 is alos reduced, so thatthe base potential of the transistor 53 is varied, and the transistors53 and 62 switch off rapidly by regenerative action. It will beappreciated that the rate at which the transistor 53 switches off isextremely rapid, and is not dependent on the rate at which the voltageis rising at the terminal 45 once the transistor 52 has started toconduct. Once the transistor 62 is off, then the potential at theterminal 46 is equal to the potential of the line 23, less the voltagedrop across two diodes, namely the diode 68 and the base-emitter diodeof the transistor 66, these diodes compensating for the diodes in thepump circuit as previously explained.

The resistors 64, 65 and 57 are selected so that the base potential ofthe transistor 53 when the transistor 62 is conducting is at apredetermined voltage above the line 23, and is at the samepredetermined voltage below the voltage of the line 23 when thetransistor 62 is off. As the input to the terminal 45 now falls, a stageis reached at which the transistor 52 conducts less, and the transistor53 starts to turn on. As soon as the transistor 53 starts to turn on, itprovides base current to the transistor 62, and by regenerative actionthe circuit again switches rapidly, independently of the rate of fall ofvoltage at the terminal 45, to the opposite state in which thetransistors 53 and 62 are on and the terminal 46 is at the potential ofthe line 21. Thus, the circuit produces a square wave output which inthis example has a mark-space ratio of approximately unity. The use ofthe transistor 54 as a constant current source permits operation over awide range of supply voltage between the lines 21, 22. The minimumamplitude of the square wave is set by the resistors 57, 64, 65.

Turning now to FIG. 9, there is shown a modification in which bothhalves of each cycles of the input at terminal 45 are used. In FIG. 9,the circuit connections are not shown in detail, but are exactly thesame as in FIG. 8, except for some additional components associated withthe transistor 52. Thus, the transistor 52 now has its collectorconnected to the line 21 by way of a resistor 81, and its collector alsoconnected to the base of a p-n-p transistor 83 with its emitterconnected to the line 21 through a resistor 84 and its collectorconnected to a terminal 46a and further connected to the collector of ann-p-n transistor 85, the base of which is connected through a diode 86to the line 23 and the emitter of which is connected through a resistor87 to the line 23. It will be seen that the transistors 83 and and theirassociated components are equivalent to the transistors 62 and 66 andtheir associated components in FIg. 8, and that the switching of thesetransistors is effected in accordance with the voltage across theresistor 81, in the same way as the switching of the transistors 62 and66 is effected by the voltage across the resistor 56 in FIG. 8. Theeffect is that the wave form at the terminal 46a is complementary to thewave form at the terminal 46. Obviously by taking an output from theterminals 46 and 46a the circuit can be made to have a more rapidresponse, or alternatively can have the same response rate for a lowerfrequency input at terminal 45.

In some examples, it is preferred that the diode 68 in FIG. 8, and itsequivalent diode 86 in FIG. 9, should have it anode connected to thejunction of a pair of resistors connected between the lines 23, 22.

FIG. 10 illustrates one form of pump circuit which is intended for usewith the arrangement of FIG. 8. Referring to FIG. 10, the terminal 46seen in FIG. 8 is connected to the line 23 through a series circuitincluding a resistor 91, a capacitor 92 and the cathode-anode part of adiode 93. The junction of the capacitor 92 and diode 93 is connectedthrough the anode-cathode path of a diode 94 to the inverting inputterminal of an operational amplifier 95 having its non-inverting inputterminal connected to the line 23. The output terminal of the amplifier95 is connected to the resistor 15d, which provides the required inputto the circuit as described with reference to FIG. 1 or FIG. 6. Thefeedback between the output terminal of the amplifier 95 and itsinverting input terminal includes a resistor 97 and a capacitor 96 inparallel and moreover the inverting input terminal of the amplifier 95is connected to the line 22 through a resistor 98.

If the capacitor 96 and its associated conventional discharge resistor97 were to be connected in parallel between the inverting input terminalof the amplifier 95 and the line 23, then the arrangement wouldconstitute a conventional diode pump circuit followed by an amplifier.With such an arrangement, the capacitor 96 would acquire a chargedependent upon the frequency of the signal at the terminal 46, but thecharge would not necessarily be directly proportional to the frequencyat the terminal 46. However, by using the capacitor 96 in the positionshown, the voltage developed across the capacitor 96 is proportional tothe frequency of the input signal at the terminal 46.

Without the resistor 98, the arrangement of FIG. 10 would produce anoutput which for zero frequency would be at the potential of the line23, and then would decrease towards the potential of the line 22 as thefrequency increased. As will be seen with reference to FIG. 2, this isnot the characteristic required, and the addition of a bias by way ofthe resistor 98 lifts the curve to the correct position, as shown inFIG. 2.

The arrangement of FIG. 11 is similar to that shown in FIG. 10, but isdesigned for use with the circuit of FIG. 9. The components 91, 92, 93and 94 are duplicated in FIG. 11, and are indicated with the samereference numerals and the suffix A. The operation is indentical to thatof FIG. 10, except tht the input frequency to the amplifier 95 isdoubled.

It is to be understood that the particular pulse shaping circuitdescribed has advantages even when used with a conventional diode pumpcircuit. Moreover, the particular pump circuit described has advantageswith or without the particular form of pulse shaping circuit.Additionally, the pump circuit described can be used in applicationsother than fuel systems.

It is a matter of some importance that the engine speed should notexceed its maximum value, becuase if it does serious damage couldresult. It will be noted that in the event of a failure in the pumpcircuit of FIG. 10 or FIG. 11 resulting in a low output voltage at theresistor 15d, then this fault will be interpreted as a high enginespeed, so that the circuit will reduce the engine speed, and no damagewill result. However, a fault resulting in a high output voltage at theresistor 15d will be interpreted as a low engine speed, and this is apotentially dangerous situation. This difficulty can be overcome bymonitoring the output of the amplifier 95, so that if the amplifierfails action is taken to prevent damage. However, one particularconvenient way of achieving the desired effect is shown in FIG. 12. Thearrangement of FIG. 12 is shown for convenience as applied to a pumpcircuit for use with the arrangement of FIG. 8.

Referring to FIG. 12, the arrangement is similar to that shown in FIG.10 with the omission of the resistor 98. In addition, however, thejunction of the resistor 91 and capacitor 92 is connected to the line 23through a capacitor 101 and the anode-cathode part of a diode 102 inseries. The junction of the capacitor 101 and diode 102 is connected tothe line 23 through the cathode-anode part of a diode 103 and acapacitor 104 in series, and the junction of the diode 103 and capacitor104 is connected through a resistor 105 to the inverting input terminalof the amplifier 95.

The operation of the arrangement shown in FIG. 12 is best explained withreference to the wave forms in FIGS. 13, 14 and 15. Without the inputfrom the resistor 105, the amplifier would have an output of the formshown in FIG. 13 (remembering that the resistor 98 is not present). Thecapacitors 101, 104 and their diodes 102, 103 form a conventional diodepump circuit producing an input to the amplifier 95 which is out ofphase with the input through the diode 94. The form of the resultantoutput of the amplifier 95 is shown in FIG. 14, and it will be seen thatit rises exponentially to a maximum value. The actual output of theamplifier 95 is the sum of the outputs shown in FIGS. 13 and 14, and isshown in FIG. 15. The output rises to a maximum voltage at the point 105and falls to zero at the point 106. The portion of the curve between thepoints 105, 106 is substantially linear, and the curve shown in FIG. 15replaces the curve shown in FIG. 2. The points 106 and 105 are thenclose to the maximum and minimum engine speeds respectively, and innormal operation the output from the amplifier 95 will be between thepoints 105, 106. It willbe seen that the form of the curve producedensures that both possible fault conditions of the amplifier 95 and thepreceding circuits result in a low voltage output, so that the circuitfails safe. It will of course be understood that the arrangement of FIG.12 can be used anywhere where it is desirable for a maximum output to beobtained at a particular frequency, this frequency being represented bythe point 105 in FIG. 15.

It is not necessary for the resistor 105 to provide an input to theamplifier 95. Where the circuit is being used with the arrangement ofFIG. 1, then if the diodes 102, 103 are appropriately connected theresistor 105 can instead provide an input to the inverting inputterminal of the amplifier 18. Similarly, where the arrangement is usedwith FIG. 6, the resistor 105 can be used to provide an input to theinverting input terminal of the amplifier 19. Because the amplifier 95and the amplifier 18 (or the amplifier 19 in FIG. 6) successively sumtheir input signals, it will be appreciated that the overall effect ofproviding the input from the second pump circuit in FIG. 12 to one ofthe amplifiers 18 or 19 does not alter the operation of the circuit.

We claim:

1. A fuel system for an engine, comprising in combination a pump supplyfuel to the engine, an actuator controlling the pump, and an electronicgovernor for controlling the actuator, said governor receivingelectrical signals representing engine speed and at least one furtherengine parameter, the speed signal being obtained using a transducerproducing an ac. output at a frequency proportional to engine speed, apump circuit for converting said signal to a dc. signal, first, secondand third supply lines, an operational amplifier having an invertinginput terminal and a non-inverting input terminal, said amplifier beingpowered by the first and second supply lines and having itsnon-inverting input terminal connected to the third supply line, andsaid pump circuit including a capacitor across which is developed avoltage proportional to the frequency of the ac. output, said capacitorbeing connected across the inverting input and output terminals of saidoperational amplifier, a resistor across the capacitor, a secondcapacitor and a diode in series between an input terminal and theinverting input terminal, and a second diode coupling the junction ofthe second capacitor and first diode to the third supply line.

2. A circuit as claimed in claim 1 including a resistor coupling theinverting input terminal of the amplifier to the second line.

3. A system as claimed in claim 1 including in addition a further pumpcircuit providing an input to the operational amplifier or directly tothe governor whereby said do. signal increases from zero to a maximumlevel at a minimum engine speed and then decreases substantiallylinearly to zero at a maximum engine speed.

4. A system as claimed in claim 1 in which the operational amplifier isbiased so that it produces a maximum output at low engine speeds,falling to zero near a maximum engine speed.

5. A system as claimed in any one of claim 1 including a shaping circuitbetween the transducer and the first mentioned pump circuit, the shapingcircuit converting the output from the transducer to substantiallysquare wave form.

6. A system as claimed in claim 5, including a pair of supply linesproviding power to the governor, said shaping circuit producing a squarewave output having an amplitude proportional to the voltage between saidsupply lines.

7. A system as claimed in claim 6 including means modifying saidamplitude to compensate for temperature dependance of components in thepump circuit.

8. A system as claimed in any one of claim 6 in which the shapingcircuit includes first and second transistors connected as a long tailedpair with a constant current source in the tail, the first transistorhaving its base connected to the transducer, and the second transistorhaving a collector-base cross coupling with a third transistor providingthe output from the pulse shaping circuit.

9. A system as claimed in any one of claim in which the shaping circuitincludes switching components producing a second square wave output outof phase with the first output, and the capacitor across the operationalamplifier is common to two pump circuits driven by the two outputs fromthe shaping circuit.

10. An electronic pump circuit in which a voltage dependent on thefrequency of an ac. input is developed across a capacitor connectedacross the input and output terminals of an operational amplifier,including first, second and third supply lines, the amplifier beingpowered by the first and second lines and having its non-inverting inputterminal connected to the third line, and the pump circuit including, inaddition to the capacitor between the output terminal and the invertinginput terminal of the amplifier, a resistor across the capacitor, asecond capacitor and a diode in series between an input terminal and theinverting input terminal, and a second diode coupling the junction ofthe second capacitor and first diode to the third line.

11. A circuit as claimed in claim 10 including a resistor coupling theinverting input terminal of the amplifier to the second line.

12. A circuit as claimed in claim 10 in combination with a conventionalpump circuit providing a further input to the amplifier, whereby withincreasing frequency the output of the amplifier rises from zero to amaximum value and then decreases with further increase in frequencytowards zero.

13. A fuel system for a compression-ignition engine, comprising incombination a pump for supplying fuel to the engine, anelectro-mechanical actuator controlling the pump, and an electronicgovernor for controlling the actuator, said governor receivingelectrical signals from three transducers representing demand, pumpoutput and engine speed, the governor including three power supplylines, the third line being kept at a specified proportion of thepotential between the first and second lines, the speed signal beingobtained using a transducer producing an ac output at a frequencyproportional to engine speed, a signal-shaping circuit to which saida.c. output is applied, said signal-shaping circuit producing a squarewave output whose amplitude is substantially proportional to thepotential between the first two supply lines, and a pump circuit forconverting said square wave output to a dc. signal which at least overthe working speed range decreases in magnitude relative to the thirdsupply line with increasing speed, said d.c. signal being applied to thegovernor, said pump circuit including a capacitor across which isdeveloped a voltage proportional to the frequency of the said a.c.output, said capacitor being connected across the input and outputterminals of an operational amplifier.

1. A fuel system for an engine, comprising in combination a pump supplyfuel to the engine, an actuator controlling the pump, and an electronicgovernor for controlling the actuator, said governor receivingelectrical signals representing engine speed and at least one furtherengine parameter, the speed signal being obtained using a transducerproducing an a.c. output at a frequency proportional to engine speed, apump circuit for converting said signal to a d.c. signal, first, secondand third supply lines, an operational amplifier having an invertinginput terminal and a non-inverting input terminal, said amplifier beingpowered by the first and second supply lines and having its noninvertinginput terminal connected to the third supply line, and said pump circuitincluding a capacitor across which is developed a voltage proportionalto the frequency of the a.c. output, said capacitor being connectedacross the inverting input and output terminals of said operationalamplifier, a resistor across the capacitor, a second capacitor and adiode in series between an input terminal and the inverting inputterminal, and a second diode coupling the junction of the secondcapacitor and first diode to the third supply line.
 2. A circuit asclaimed in claim 1 including a resistor coupling the inverting inputterminal of the amplifier to the second line.
 3. A system as claimed inclaim 1 including in addition a further pump circuit providing an inputto the operational amplifier or directly to the governor whereby saidd.c. signal increases from zero to a maximum level at a minimum enginespeed and then decreases substantially linearly to zero at a maximumengine speed.
 4. A system as claimed in claim 1 in which the operationalamplifier is biased so that it produces a maximum output at low enginespeeds, falling to zero near a maximum engine speed.
 5. A system asclaimed in any one of claim 1 including a shaping circuit between thetransducer and the first mentioned pump circuit, the shaping circuitconverting the output from the transducer to substantially square waveform.
 6. A system as claimed in claim 5, including a pair of supplylines providing power to the governor, said shaping circuit producing asquare wave output having an amplitude proportional to the voltagebetween said supply lines.
 7. A system as claimed in claim 6 includingmeans modifying said amplitude to compensate for temperature dependanceof components in the pump circuit.
 8. A system as claimed in any one ofclaim 6 in which the shaping circuit includes first and secondtransistors connected as a long tailed pair with a constant currentsource in the tail, the first transistor having its base connected tothe transducer, and the second transistor having a collector-base crosscoupling with a third transistor providing the output from the pulseshaping circuit.
 9. A system as claimed in any one of claim 5 in whichthe shaping circuit includes switching components producing a secondsquare wave output out of phase with the first output, and the capacitoracross the operational amplifier is common to two pump circuits drivenby the two outputs from the shaping circuit.
 10. An electronic pumpcircuit in which a voltage dependent on the frequency of an a.c. inputis developed across a capacitor connected across the input and outputterminals of an operational amplifier, including first, second and thirdsupply lines, the amplifier being powered by the first and second linesand having its non-inverting input terminal connected to the third line,and the pump circuit Including, in addition to the capacitor between theoutput terminal and the inverting input terminal of the amplifier, aresistor across the capacitor, a second capacitor and a diode in seriesbetween an input terminal and the inverting input terminal, and a seconddiode coupling the junction of the second capacitor and first diode tothe third line.
 11. A circuit as claimed in claim 10 including aresistor coupling the inverting input terminal of the amplifier to thesecond line.
 12. A circuit as claimed in claim 10 in combination with aconventional pump circuit providing a further input to the amplifier,whereby with increasing frequency the output of the amplifier rises fromzero to a maximum value and then decreases with further increase infrequency towards zero.
 13. A fuel system for a compression-ignitionengine, comprising in combination a pump for supplying fuel to theengine, an electro-mechanical actuator controlling the pump, and anelectronic governor for controlling the actuator, said governorreceiving electrical signals from three transducers representing demand,pump output and engine speed, the governor including three power supplylines, the third line being kept at a specified proportion of thepotential between the first and second lines, the speed signal beingobtained using a transducer producing an a.c. output at a frequencyproportional to engine speed, a signal-shaping circuit to which saida.c. output is applied, said signal-shaping circuit producing a squarewave output whose amplitude is substantially proportional to thepotential between the first two supply lines, and a pump circuit forconverting said square wave output to a d.c. signal which at least overthe working speed range decreases in magnitude relative to the thirdsupply line with increasing speed, said d.c. signal being applied to thegovernor, said pump circuit including a capacitor across which isdeveloped a voltage proportional to the frequency of the said a.c.output, said capacitor being connected across the input and outputterminals of an operational amplifier.